Patterned thin films of conductive materials are used to form interconnects between different structures of an integrated circuit. The primary requirement for these interconnects is that they be made of a high conductivity material. For example, in silicon based semiconductor devices, metallic films of aluminum, copper and their alloys have been used as interconnects.
When defining the resistance of the interconnect system it is important to consider, in addition to the intrinsic conductivity of the metal, the ability of the metal to make low resistance contacts to the semiconductor. Good contact necessitates an intimate chemical proximity that easily permits the transport of electrons across the interface. Intimate chemical proximity, however, could imply a propensity for a chemical reaction that could, over time or under severe conditions, degrade the performance of the device. Such degradation is seen when aluminum alloy interconnects are used in direct contact with doped silicon surfaces. The degradation, enhanced at high temperatures, takes the form of "junction spiking", a phenomenon in which the silicon dissolves in and is transported through the aluminum. The aluminum in return, "spikes" into the silicon to fill the void left behind by the consumed silicon. To minimize the interaction, the concept of diffusion barriers has been invoked. These are thin layers of conductive films that are placed between the interconnect and the underlying semiconductor. Examples of diffusion barriers in the Al alloy-Si system are titanium-tungsten (TiW), titanium nitride (TIN) and tungsten silicide (WSi.sub.x). These diffusion barriers maintain electrical continuity while metallurgically separating the interconnect from the semiconductor.
As the density of the integrated chips has increased, so have the demands on the metallization schemes used. This is particularly emphasized at the contact. The following are requirements of high density integrated chips:
(a) Contacts have had to become smaller. This has increased the resistance per contact. First order scaling theory predicts that contact resistance increases inversely with the area of the contact, i.e., with the square of the critical dimension. PA1 (b) "Scaled devices" also require that the metallurgical depths of the contact be shallower. This requirement further increases the need for diffusion barriers, as now even a lesser amount of spiking may ruin the junction below the contact. PA1 (a) providing an n.sup.+ region by a dopant in the silicon substrate wherein the dopant concentration is greater than 5.times.10.sup.15 /cm.sup.2 ; PA1 (b) forming an undoped WSi.sub.x film in contact with the n.sup.+ region such WSi.sub.x film having a thickness between 1000 .ANG. and 2500 .ANG. and wherein x is defined by the relationship 2&lt;x&lt;3 and the film/n.sup.+ contact area is less than 1.7 .mu.m.sup.2 ; PA1 (c) annealing such WSi.sub.x layer to activate the n.sup.+ region; and PA1 (d) providing a metal film on the annealed WSi.sub.x film.